The present invention relates to a color filter and a liquid crystal display device including a combination of the color filter and a specific backlight unit.
A color liquid crystal display device basically has a first transparent substrate having a first transparent electrode layer formed thereon, a second transparent substrate having a second transparent electrode layer formed thereon, and a liquid crystal layer filled between the first and second transparent substrates. A color filter layer is generally interposed between the second transparent substrate and the second transparent electrode layer. A first polarizer and a second polarizer are disposed outside the first and second transparent substrates, respectively. A backlight unit having a backlight source is disposed outside the first polarizer.
In the operation of such a liquid crystal display device, the electric voltage applied across these first and second transparent electrode layers is adjusted at every pixel so as to control the degree of polarization of light from the backlight unit which has passed through the first polarizer. In this way, the quantity of light passing through the second polarizer is controlled in performing the display of the display device.
Therefore, the emission spectrum of the backlight and the spectral characteristics of the color filter are important factors in the determination of the color characteristics of the color liquid crystal display device.
A pigment dispersion type color filter, which is excellent in various resistances such heat resistance and light resistance, has been conventionally employed as a color filter. On the other hand, a backlight unit having, as a light source, a three-band fluorescent lamp (hereinafter sometimes referred to as a three-band lamp), which is excellent in color rendering property, has been extensively employed. Due to the development of these pigment dispersion type color filter and backlight unit exhibiting excellent color rendering property, the color liquid crystal display device has been widely placed on the market as a liquid crystal color television, a display device for car navigation system, a liquid crystal display-integrated notebook personal computer, etc. Furthermore, by taking advantages of the color liquid crystal display device in terms of energy saving and space saving, the color liquid crystal display device is now increasingly employed as a monitoring device for a desktop personal computer, and hence is now noticed as a substitute display device for the conventional CRT monitoring device.
However, at present, the display performance of the CRT is still superior to that of the liquid crystal display device. In particular, it is still failed to develop a liquid crystal display device which meets the EBU (European Broadcasting Union) Standard, which is a European standard for the display colors of the CRT. Therefore, the achievement of this standard is the key to a big propagation of the liquid crystal display device in the field of television or multimedia industries.
In the operation of a color television, the configuration, movement and hue of an image subject are reproduced on a picture screen through a sequence of processes, i.e. (1) picturing (using a color camera); (2) image transmission; and (3) image reproducing (using an image-receiving device). Therefore, the image transmission system of image signals including the hue is standardized. One of representative image transmission systems is NTSC (National Television System Committee) system, which is adopted, as a television broadcasting system, mainly in the U.S.A., Canada and Japan. On the other hand, in European countries, the image transmission system and standards are determined by the EBU.
The factor which determines the color-reproducing zone in a color television is the chromaticity of three primary colors (display primaries) of the receiver unit, so that the spectral characteristics which a color camera should have will be also determined by this chromaticity. According to the NTSC system, the image-receiving side three primary colors are stipulated with respect to the chromaticity coordinates x and y in the XYZ color specification system as follows:
Red: x=0.67; y=0.33 PA1 Green: x=0.21; y=0.71 PA1 Blue: x=0.14; y=0.08 PA1 Red: x=0.64; y=0.33 PA1 Green: x=0.29; y=0.60 PA1 Blue: x=0.15; y=0.06
On the other hand, according to the EBU Standard, the image-receiving side three primary colors are stipulated as follows:
Note that x=X/(X+Y+Z); y=Y/(X+Y+Z); and X, Y and Z are tristimulus values in the XYZ color specification system.
The chromaticity of the color liquid crystal display device which is actually employed now however is based on the NTSC system. As mentioned above, there is no liquid crystal display device put to practical use which satisfies the chromaticity stipulated by the EBU Standard. Specifically, as far as red and green colors are concerned, it is relatively easy to achieve the EBU Standard by using color filters prepared from conventional pigment dispersion type colored compositions in combination with a backlight from a conventional three band lamp. With respect to blue however, there is a problem that if the EBU Standard about blue color is to be achieved by using a blue color filter prepared from a conventional blue-colored composition containing copper phthalocyanine pigment in combination with a backlight from a conventional three band lamp, the film thickness of the blue color filter must be increased to a considerably large extent. Since the film thickness of the usually used color filters is generally in the range of 1 to 2.5 .mu.m, such an increase in film thickness of the blue color filter is not practical.
More specifically, if a blue color filter which is capable of satisfying the EBU Standard is to be formed by making use of the conventional blue-colored composition for the purpose of manufacturing a color liquid crystal display comprising a backlight unit having, as a light source, the conventional three band lamp, the film thickness of the blue color filter is required to be increased to 5 to 9 .mu.m. This is inappropriate for the following reasons.
Generally, if the thickness of the color filter is too thin, a sufficient chromaticness can not be obtained. On the other hand, if the thickness is too thick, the shape of the patterned filter is degraded. In the case of using a light-sensitive pigment-dispersed resist, the thick filter requires a prolonged time for dissolving and removing the portions not photo-cured, with the result that the periphery of the pattern is laterally etched away, degrading the linearity. Further, a light-sensitive pigment-dispersed resist itself has a large light absorption capacity. Therefore, the lower portion of the applied resist tends to remain uncured when the applied resist is exposed to light in patterning because the light does not sufficiently reach the lower portion. Thus, the thickness of the uncured portion is increased when the thickness of the applied resist is increased, bringing about a larger amount of lateral etching. As a result, the cross-section of the resultant filter becomes inversely tapered. If the color filter has the inversely tapered cross-section, a transparent electrode formed thereon tends to become discontinuous, resulting in a display failure. Further, if the first color filter segment or the second color segment has the inversely tapered cross-section, a subsequently formed color filter segment tends to become nonuniform. When the exposure time is shortened to suppress the lateral etching, the unexposed portion is not sufficiently removed, leaving a residue, which allows only small latitude of development. For these reasons, an appropriate film thickness of the color filter is 1.0-2.5 .mu.m, more suitably 1.0-2.0 .mu.m.